The E949 collaboration at the Brookhaven National Laboratory has reported further evidence for the very rare kaon decay, K+→π+ννbar. The rate observed for this decay may indicate new forces beyond those in the Standard Model – which predicts the frequency of such decays to be half that observed – although it is still too soon to say if a deviation has occurred.
The decay K+→π+ννbar is extremely important due to its sensitivity to the mixing strength Vtd of the t quark to d quark, which is poorly known, and to many hypothetical new physical effects not accounted for in the Standard Model. |Vtd| can be extracted from the K+→π+ννbar branching ratio with minimal theoretical uncertainty, since the hadronic matrix element can be extracted from the well measured K+→π0e+ν decay and higher order corrections have been calculated by Andrzej Buras, Gerhard Buchalla and others.
The new result on K+→π+ννbar comes from the E949 experiment – an upgraded version of E787, which reported two earlier sightings of the same decay. The two experiments, run by a collaboration of some 70 scientists from Canada, Japan, Russia and the US, have taken place at Brookhaven’s Alternating Gradient Synchrotron (AGS), the world’s highest intensity proton synchrotron. The improved apparatus of E949 has exploited higher beam intensities and achieved greater detection efficiency than any previous experiment of this type.
Although a neutrino and an antineutrino are emitted in the process K+→π+ννbar, these particles interact too weakly to be detected. Thus, evidence that one positive pion – and only one positive pion – was produced by the kaon decay had to be proved beyond reasonable doubt, eliminating the possibility that other detectable particles were present. This required the most efficient particle detector system ever built, as well as analysis techniques capable of confirming the required suppression. For example, the detection efficiency achieved for neutral pions was such that fewer than one in a million were missed. To establish the validity of the observations, all backgrounds had to be suppressed by a factor of 1011. This was among the first modern analysis efforts to apply carefully “blind” or unbiased analysis techniques, which are now standard practice in high-energy physics.
Out of all the data analysed, involving nearly 1013 kaons, three events explicable by the decay K+→π+ννbar have now been seen by E787 and E949. This indicates that the K+→π+ννbar process occurs with a branching ratio of 1.47+1.30-0.89 x 10-10, making it one of the rarest particle decays ever observed. The result continues to suggest a possible discrepancy with the Standard Model, although with only three events it is still consistent with the prediction of (7.7±1.1) x 10-11.
The goal of E949 was to increase the experimental exposure of E787 by five times. If the E949 findings for K+→π+ννbar were to continue at the current pace, 20 or more events would be observed. Such a result could alter our current picture of particle physics, forcing an expanded view of the fundamental constituents of the universe and their interactions. The detector and collaboration are ready to complete the experiment; however, further running is currently not possible because the US Department of Energy discontinued high-energy physics operations at the AGS in 2002, before E949 was completed.
Future work on the related process K0L→π0ννbar, supported by the US National Science Foundation, is now getting going, with the construction of the KOPIO experiment due to begin at the AGS next year.
